The invention relates to novel colloid dispersion mixtures comprising a water-soluble monomer and a water-insoluble comonomer, to processes for their preparation and to their use as protective colloid for emulsion polymerization in water.
Protective colloids are normally water-soluble polymer compounds which are employed for conducting an emulsion polymerization and then for stabilizing the finely dispersed polymer particles.
Typical protective colloids are, for example, polyvinyl alcohols or cellulose derivatives. Protective colloids, owing to their structure, are in part incorporated into the polymer as it forms in the course of emulsion polymerization. In this case one speaks of a graft polymerization. As a result, dispersions stabilized by protective colloid are often much more stable to mechanical loads than are emulsifier-stabilized dispersions.
In aqueous polymer dispersions it is normal to use as protective colloids very hydrophilic polymers, such as poly(vinyl alcohol), poly(ethylene oxide) or poly(vinylpyrrolidone) (Gosh, S.; Krishnamuti, N., Paint & Ink International, Jan-Feb. 19, 1996). Other synthetic copolymers which have also been used as protective colloids are poly(alkyl-oxazolines), U.S. Pat. No. 5,124,394, and poly(N-vinylformamide), U.S. Pat. No. 5,470,903. However, owing to the strong hydrophilicity of these materials, films of these polymer dispersions are very sensitive to water. This sensitivity to water greatly restricts the fields of use of such materials. For instance, materials of this kind cannot be used for exterior applications since the polymer is degraded very rapidly by weathering.
For practical applications, therefore, it is a great problem to achieve the right balance in the hydrophilicity of the protective colloids such that, on the one hand, the emulsion polymerization can be readily carried out and, on the other hand, the polymers prepared therewith possess the necessary water resistance.
Protective colloids are also utilized in order to achieve very specific rheological properties of dispersions: for example, a defined yield point. These rheological properties are very important if, for example, such copolymer dispersions are used as components of synthetic-resin plasters.
It is an object of the present invention to provide novel protective colloids which permit the preparation of mechanically stable copolymer dispersions having particular rheological properties.
The object has been achieved by a colloid dispersion mixture obtained by copolymerizing a water-soluble monomer with a water-insoluble comonomer in the presence of a micelle-forming emulsifier.